Synthetic resins have been used widely in various areas, and a flame retardant is generally added to flammable resins for providing flame resistance. In particular, currently when the use amount of a self-extinguishing flame-resistant resin polyvinyl chloride is declining because of its adverse effects on environment, the use amounts of flammable resins such as polyethylene, polypropylene, and polystyrene are increasing as the replacement, which, in turn, is leading to increased demand for flame retardants. In addition, a flame retardant is occasionally added even to flame-resistant synthetic resins for improvement in flame resistance.
Thus, various flame retardants have been provided. The flame retardants used include phosphorus-based retardants (Japanese Patent Application Laid-Open (JP-A) No. 2002-80633), halogen-based retardants (JP-A No. 8-291128), inorganic hydrates such as aluminum hydroxide (JP-A No. 2002-338816), magnesium hydroxide (JP-A No. 2003-3171), and boehmite (JP-A No. 2002-2091); and the like. However, phosphorus-based flame retardants have an environmental problem, for example, of causing aquatic eutrophication, while halogen-based flame retardants of generating smoke and hazardous gases during combustion or incineration, which are attracting attention recently as a possible cause of serious fire deaths, and thus, there exists a need for inorganic flame retardants having no such drawbacks. Among inorganic flame retardants, particularly aluminum hydroxide (Al(OH)3) is rich in constitutional water, superior in flame-retarding effect and in acid and alkali resistances, and also advantageous from the point of cost, and thus are widely used. As for the flame-retarding properties of aluminum hydroxide, dehydration is known to start gradually at approximately 200° C. and progress rapidly at around 230° C. to 250° C.
However, many thermoplastic resins have a molding temperature at around the dehydration temperature of aluminum hydroxide, and dehydration of aluminum hydroxide during molding often resulted in drop in yield because of the bubbles generated in the synthetic resin molded products and the irregularity on the surface thereof. Although thermosetting resins are often molded at a temperature lower than that of thermoplastic resins, these resins are occasionally used at a higher temperature because of the inherent properties of the synthetic resins, for example, as the parts in electric/electronic devices, and aluminum hydroxide therein occasionally resulted in dehydration depending on the environment temperature used, leading to decrease in the yield and deterioration in the physical properties of molded products. For example when a thermosetting resin is used as an electronic board, the environment temperature of the electronic board reaches as high as approximately 230° C. during soldering, leading to dehydration of aluminum hydroxide and consequently to decrease in the yield of the electronic board.
Under the circumstances, it would be possible to use other inorganic flame retardants having a higher dehydration temperature. However, for example, boehmite (AlO(OH)), which has a dehydration peak of around 500° C., may seem advantageous, but has a drawback of containing a smaller amount of constitutional water. Alternatively, magnesium hydroxide (Mg(OH)2), which has a high dehydration peak temperature of about 380° C., is strongly alkaline, facilitating decomposition of the synthetic resin, and unstable to acids, and thus had problems, for example, of prohibiting use thereof in an environment for example in contact with acid and easier solubilization thereof during etching of electronic device parts with acid. Magnesium hydroxide also had a drawback in that it is converted to its basic carbonate salt under high-humidity condition by absorption of the carbon dioxide gas in air, causing a whitening phenomenon of the resin surface containing the flame retardant magnesium hydroxide and affecting the properties of the product.
Accordingly, an aluminum hydroxide flame retardant having a higher dehydration temperature and a sufficiently large amount of dehydration water is most desirable.